Two-wire ballast for fluorescent tube dimming

ABSTRACT

A two-wire ballast arrangement is disclosed for fluorescent tubes having first and second terminals for supplying variable power to dim the fluorescent tube, a choke coil connected between the first terminal and the fluorescent tube and a filament transformer having a primary connected between the second terminal and a tap on the choke coil, the tap being selected to supply substantially constant voltage to the primary winding, the transformer having secondary windings to supply filament voltage to the fluorescent tube.

BACKGROUND OF THE INVENTION

This invention relates to a ballast arrangement for fluorescent tubesand particularly to a two-wire ballast arrangement to supplysubstantially constant filament voltage to a fluorescent tube suppliedby a variable power source for dimming.

Typical fluorescent tubes comprise a sealed cylinder of glass having aheating filament at either end and filled with a gas such as mercuryvapor. The filaments are heated by a supplied voltage to causethermionic emission so that an arc can be struck across the tube causingthe gas to radiate. The radiation given off by gases such as mercury isshort-wave ultraviolet radiation and thus produces little visible light.In order to provide visible light, the inside of the tube is coated witha suitable phosphor which is activated by the ultraviolet radiation andemits visible light of a color that is characteristic of theparticularly phosphor or mixture of phosphors employed to coat the tube.In order to sustain the arc across the tube, the filament voltage mustbe maintained to a predetermined level which poses a problem when thefluorescent tube is to be used in a light dimming arrangement.

As one answer to this problem, the prior art developed a three-wiresystem where the filament voltage was separated from the variablecurrent used to dim the fluorescent tube. In such a system, a first wirewas used to supply variable current to the fluorescent tube. A secondwire was used to supply constant filament voltage to the filaments ofthe tube. The third wire was used as a return. From a cost ofinstallation standpoint, a two-wire arrangement for dimming fluorescenttubes is preferable.

The prior art has developed two-wire fluorescent tube dimmingarrangements but these provide only a limited dimming capability. Theproblem with the prior art two-wire dimming arrangements is that thefilament voltage to the tube is varied as the dimming current supply tothe fluorescent tube is varied. Thus, as the current supplied to thefluorescent tube is reduced to dim the tube, the filament voltage isalso reduced resulting in stripping the emission-coating from thecathodes, and when the filament voltage has been reduced below the levelto sustain the arc across the tube, the tube will extinguish. Thepresent invention extends the dimming range of the tube by supplyingsubstantially constant filament voltage to the tube in a two-wiresystem.

SUMMARY OF THE INVENTION

In the present invention, first and second terminals provide variabledimming power to the fluorescent tube, a choke coil is connected betweenthe first terminal and the tube and a transformer is provided having aprimary winding connected between the second terminal and a tap on thechoke coils selected to provide substantially constant voltage to theprimary winding even as the power to the tube is varied, the transformerfurther having secondary windings associated with the primary winding tosupply filament voltage to the filaments of the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will becomeapparent from a detailed consideration of the invention when taken inconjunction with the drawings in which:

FIG. 1 shows a three-wire prior art arrangement for dimming afluorescent tube;

FIG. 2 shows a prior art two-wire arrangement for dimming a fluorescenttube;

FIG. 3 shows the voltage vs. the controlled current through the tube foreach side of the choke coil of FIG. 2 and for the tap on the choke coilof FIG. 4;

FIG. 4 shows the ballast arrangement according to the instant invention;

FIG. 5 shows an arrangement of the present ballast for supplying twofluorescent tubes;

FIG. 6 shows an alternative ballast arrangement for supplying two tubesaccording to the instant invention;

FIG. 7 is another variation for two-wire control;

FIGS. 8 and 9 show alternative ballast arrangements according to theinstant invention.

DETAILED DESCRIPTION

The fluorescent dimming arrangement 10 shown in FIG. 1 is a three-wiresystem comprising wires 11, 12 and 13 connected to respective inputterminals 14, 15 and 16. First wire 11 connects terminal 14 to chokecoil 17 the other side of which is connected to fluorescent tube 18 byline 19. Line 20 connects the other side of tube 18 to terminal 16.Connected across terminals 15 and 16 is primary winding 21 of filamenttransformer 22. Terminal 14 supplies variable power or current tofluorescent tube 18 for dimming and transformer 22 connected toterminals 15 and 16 supplies constant filament voltage to fluorescenttube 18. Thus, secondaries 23 and 24 of transformer 22 are connected torespective filaments within fluorescent tube 18. The starting stripe 25which is accomplished by the fixture is associated with fluorescent tube18 and is connected to ground terminal 26 by line 27.

Since the filaments of the fluorescent tube must be maintained at theirspecified voltage to keep them at their temperature and to sustain anarc across the tube, the prior art recognized that a constant voltagesource for the filaments of tube 18 was desirable to increase the rangeover which it could be dimmed as controlled by the variable power orcurrent supplied by terminal 14. To this end, the prior art arrangementas shown in FIG. 1 comprised a separate filament voltage source(connected to terminals 15 and 16), not associated with the variablesupply terminal 14, for supplying constant voltage to the tubes'filaments. However, this system is a three-wire system and the prior arthas also recognized that a two-wire system would reduce the cost andcomplexity of installation. Thus, the prior art developed the type ofsystem shown in FIG. 2.

In FIG. 2, first wire 30 connects variable current or power sourceterminal 31 to choke coil 32 the other side of which is connected byline 33 to fluorescent tube 34. The other side of tube 34 is connectedby line 35 and by second wire 37 to second supply terminal 36. Connectedbetween first wire 30 and second wire 37 is primary winding 38 offilament supply transformer 39. Transformer 39 has secondary winding 40connected to one filament of tube 34 and second secondary winding 41connected to the other filament of tube 34. The starting stripe 42associated with tube 34 is connected by line 43 to ground terminal 44.This arrangement, although it reduces the cost and complexity ofinstallation of a dimming system for controlling the intensity of afluorescent tube, does not provide a system for supplying constantfilament voltage to tube 34 and, therefore, the operating range for thedimming of tube 34 is very limited.

As can be seen from FIG. 3, as the current through the choke coil 32 isvaried from 0% to 100% by variable supply terminal 31, the voltage atpoint B increases whereas the voltage at point E on the opposite side ofthe choke coil decreases due to the negative resistance behaviour of thetube. Thus, in the arrangement of FIG. 2, as the dimming currentsupplied to tube 34 decreases, the voltage supplied to the filaments oftube 34 also decreases.

In order to avoid the problems associated with the circuit of FIG. 2 butyet retain the advantages of a two-wire system, the present inventionconnects the primary winding of the filament supply transformer to a tapon the choke coil selected to give relatively constant voltage. In FIG.3, the voltage V_(T) represents the voltage at the tap of the choke coilwhich is selected to be connected to the primary of the filament supplytransformer and is a constant or substantially constant voltage as thedimming current is varied.

In FIG. 4, choke coil 50 of ballast 51 has a tap T selected to producethis substantially constant voltage. First wire 53 connects choke coil50 to variable current or power supply terminal 52. Primary winding 54is connected between tap T on choke 50 and second wire 56. Filamentsupply transformer 56 has secondary coil 58 connected across filament 59of fluorescent tube 60. Filament 59 is also connected by line 61 to theother side of choke coil 50. A second secondary winding is comprised oftap 62 of primary winding 54 which tap is connected to one side offilament 63 of fluorescent tube 60. The other side of filament 63 isconnected by line 64 and second wire 56 to terminal 55. Provision ismade for starting stripe 65 connected by line 66 to ground terminal 67.

Since the voltage at tap T on choke coil 50 is substantially constant asthe current therethrough varies, the filament transformer 57 will supplysubstantially constant voltage to filaments 59 and 63 of fluorescenttube 60. In this manner, the operating range of the fluorescent tube isgreatly extended without decreasing its life since the filament voltageand emission temperature will be maintained through the whole dimmingrange.

In order to dim fluorescent tube 60, a dimming circuit 70 is providedconnected between a source of alternating current and terminals 52 and55. Dimming circuit 70 is comprised of a solid-state semiconductorswitch or triac 71 having one side connected to one side of thealternating current source and the other side to terminal 52. Connectedacross triac 71 is a series combination of variable resistance 72 andcapacitor 73. A diac 74 is connected from the junction of variableresistance 72 and capacitor 73 to the gate terminal of triac 71.Resistor 75 is connected from the junction of triac 71 and terminal 52to the junction of the other side of the alternating current source andterminal 55. The dimming control circuit 70 is a phase control circuitwhich controls the amount of current supplied to terminal 52 by thesetting of variable resistance 72.

FIG. 5 shows a variation of the circuit of FIG. 4 for supplying twofluorescent tubes 80 and 81. First wire 82 connects a variable power orcurrent source terminal 83 to choke coil 84 of ballast 85. The other ofthe choke coil is connected to fluorescent tube 80. Primary winding 86of filament supply transformer 87 is connected from tap T on choke coil84 to second supply terminal 88 by second wire 89. Filament supplytransformer 87 has first secondary winding 90 connected to a firstfilament of fluorescent tube 80, a second secondary winding 91,comprised of a tap on primary winding 86, connected to a first filamentof fluorescent tube 81 and a third secondary 92 connected to the secondfilament of both fluorescent tubes 80 and 81. The starting stripe 93associated with fluorescent tube 80 and starting stripe 94 associatedwith fluorescent tube 81 are connected to ground terminal 95 by line 96.Since tap T is selected as the substantially constant voltage point onchoke coil 84, secondary windings 90, 91 and 92 will supplysubstantially constant filament voltage to fluorescent tubes 80 and 81.

The system in FIG. 6 is substantially similar to the circuit of FIG. 5and, therefore, like elements have been assigned like referencenumerals. The primary difference between these two circuits is thatinstead of secondary winding 91 of FIG. 5 being comprised of a tap onprimary winding 86, a separate secondary winding 91' associated withtransformer 87 is provided for the first filament of fluorescent tube81. Also, a small, start-aid capacitor 100 is connected from the top ofsecondary winding 90 to the top of secondary winding 92.

Finally, the system of FIG. 7 is designed to operate from low voltage(e.g. 200-277 volts) two-wire supply mains 110 and 111. The step-up autotransformer 112 provides high voltage (e.g. 347 voltage) thereacross forsupplying filament heating voltage to secondaries 113, 114 and 115 fortubes 116 and 117.

The arrangements shown in FIGS. 8 and 9 include a choke coil arrangementwhich is equivalent to the tapped choke coil arrangements shown in FIGS.4-7. Specifically, a choke coil transformer 120 has a first winding 121connected between terminal L1 and fluorescent tube 122. Second winding123 of the transformer 120 is connected between terminal L1 and one sideof primary winding 124, of transformer 150, the other side of which isconnected to terminal L2. One filament of fluoresecent tube 122 issupplied with filament voltage from primary winding 124 by way ofsecondary winding 125 whereas the second filament of fluorescent tube122 is supplied by secondary winding 126 of transformer 150. Onefilament of fluorescent tube 127 is supplied with voltage from secondarywinding 126 and the other filament is supplied by secondary winding 128of transformer 150. The lower lead of secondary winding 128 is connectedto terminal L2.

By way of example, if the winding 84 shown in FIG. 6 has a total of2,000 turns with 500 turns on the left side of the tap and 1,500 turnson the right side of the tap, then winding 121 will have a total of2,000 turns and winding 123 will have 500 turns. In this manner, thenumber of turns between windings 121 and 123 are selected, as is the tapin FIG. 6, to provide a constant filament voltage by way of primarywinding 124 and secondary windings 125, 126 and 128.

In FIG. 9, the choke coil transformer 130 is essentially the same aschoke coil transformer 120 of FIG. 8 except that the second winding 132of transformer 130 is connected to the right side of first winding 131instead of to the left side.

Thus, if winding 123 should have 500 turns when winding 121 has 2,000turns, winding 132 of FIG. 9 should have 1,500 turns when winding 131has 2,000 turns. Thus, the number of turns that the second winding ofthe choke coil transformer has is dependent not only upon the number ofturns of the first winding but also upon the side of the first windingto which the second winding is connected. Grounding stripes may beprovided as are shown in FIGS. 5 and 7 where necessary.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. A ballast for a fluorescent tubedimming arrangement comprising:first and second terminals adapted to beconnected to a source of variable power; an inductive coil connected tosaid first terminal and adapted to be connected to a fluorescent tube;and, a transformer having a primary winding, said primary winding havinga first end connected to a tap on said inductive coil selected so that,as the power supplied to said fluorescent tube is varied for dimming,the voltage across said primary winding remains substantially constant,and a second end connected to said second terminal, said transformerfurther having first and second secondary windings for supplyingfilament voltage to said fluorescent tube.
 2. The ballast of claim 1wherein said transformer comprises a third secondary filament heatingwinding for supplying a second fluorescent tube with variable dimmingpower.
 3. A two-wire ballast system for a fluorescent tube dimmingarrangement comprising:first and second terminals for supplying variabledimming power to said fluorescent tube; first connecting means includinga choke coil connecting said first terminal to a first side of saidfluorescent tube; second connecting means connecting a second side ofsaid fluorescent tube to said second terminal; and, constant filamentvoltage supply means connected .[.between said first connecting meansand said second connecting means and connected to said filaments of saidfluorescent tube.]. .Iadd.to a tap on said choke coil selected so that,as the power supply to said fluorescent tube is varied for dimming, thevoltage supplied by said constant filament voltage supply means remainssubstantially constant, said constant filament voltage supply meansconnected to said second terminal and having output means .Iaddend.forsupplying constant filament voltage .[.thereto.]. .Iadd.to saidfluorescent tube.Iaddend..
 4. The two-wire ballast system of claim 3wherein said constant filament voltage supply means comprises atransformer having a primary winding, said primary winding having afirst end connected to a tap on said choke coil selected so that, as thepower supply to said fluorescent tube is varied for dimming, the voltageacross the primary winding will remain substantially constant, and asecond end connected to said second terminal, said transformer furtherhaving first and second secondary windings for supplying filamentvoltage to said fluorescent tube.
 5. The two-wire ballast system ofclaim 4 wherein said first secondary winding comprises a separatewinding magnetically coupled to said primary winding and connected to afirst filament of said fluorescent tube and said second secondarywinding comprising a tap on said primary winding and a second filamentof said fluorescent tube connected across said tap on said primarywinding and said second terminal.
 6. The two-wire ballast system ofclaim 5 further comprising a grounded starting stripe associated withsaid fluorescent tube.
 7. The two-wire ballast system of claim 4 furthercomprising a second fluorescent tube and wherein said transformercomprises a third secondary winding, said fluorescent tube and saidsecond fluorescent tube each having first and second filaments, saidfirst secondary winding connected t said first filament of saidfluorescent tube, said third primary winding connected to said firstfilament of said second fluorescent tube and said second secondarywinding connected to said second filaments of said fluorescent tube andsaid second fluorescent tube.
 8. The two-wire ballast system of claim 7wherein said first secondary winding comprises a separate windingmagnetically coupled to said transformer, said second winding comprisesa separate winding magnetically coupled to said transformer and saidthird secondary winding comprises a tap on said primary winding.
 9. Thetwo-wire ballast system of claim 8 comprising a first grounded startingstripe associated with said fluorescent tube and a second groundedstarting stripe associated with said second fluorescent tube.
 10. Thetwo-wire ballast system of claim 7 wherein said first secondary windingcomprises a separate winding magnetically coupled to said primarywinding, said second winding comprising a separate winding magneticallycoupled to said primary winding and said third secondary windingcomprising a separate winding magnetically coupled to said primarywinding.
 11. The two-wire ballast system of claim 10 comprising a firstgrounded starting stripe associated with said fluorescent tube and asecond grounded starting stripe associated with said second fluorescenttube.
 12. The two-wire ballast system of claim 3 further comprising adimming control circuit connected between a source of alternatingcurrent and said first and second terminals.
 13. The two-wire ballastsystem of claim 2 wherein said transformer is a step-up autotransformer.
 14. The two-wire ballast system of claim 13 wherein saidauto transformer has a first secondary winding connected to a firstfilament of a first tube, a second secondary connected to a firstfilament of a second tube, and a third secondary winding connected tosecond filaments of said first and second tube. .[.
 15. The two-wireballast of claim 3 wherein said constant filament voltage supply meanscomprises a secondary coil having a first end connected to one side ofsaid choke coil and a second end connected to said filament of saidfluorescent tube whereby the number of turns of said choke coil and saidsecondary coil are chosen so that said filament supply circuit willsupply substantially constant voltage to said filament..]. .[.16. Thetwo-wire ballast of claim 15 wherein said choke coil has a first endconnected to said first terminal and a second end connected to saidfluorescent tube and wherein said first end of said secondary winding isconnected to said first end of said choke coil..]. .[.17. The two-wireballast of claim 16 wherein said filament supply circuit comprises aprimary winding connected between said second end of said secondarywinding and said second terminal and having a first secondary windingconnected to one filament of said fluorescent tube and a secondsecondary winding connected to a second filament of said fluorescenttube..]. .[.18. The two-wire ballast of claim 15 wherein said choke coilhas a first end connected to said first terminal and a second endconnected to said fluorescent tube and wherein said secondary coil has afirst end connected to said second end of said choke coil and whereinsaid filament supply circuit comprises a primary winding connectedbetween said second end of said secondary coil and said second terminal,said primary winding having a first secondary winding associatedtherewith connected to a first filament of said fluorescent tube and asecond secondary winding associated therewith connected to a secondfilament of said fluorescent tube..].